I am going to explain the signaling specificity between pheromone pathway and high-osmolarity pathway in yeast, reported by Patterson et al. (Sci. Signal. 2010). McClean et al. (Nat. Genet. 2007) already reported the mathematical model, and they insisted the specificity was achieved by “mutual inhibition”. However, Patterson et al. denied it by experiment. And so, I will go back to the model of McClean et al. and explain the result of Patterson et al.